MAN: Mention that the services' list is optional
    
    For platforms where systemd is supported, after making the responders
    socket-activatable, the services' list is completely optional. So, let's
    mention that in the manual page for sssd.conf, including instructions on
    how to enabled the desired services.
    
    Related:
    https://fedorahosted.org/sssd/ticket/2243
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>